Solar cells have been noted and expected as a clean energy source, but their cost reduction is indispensable for their spread. It has, therefore, been earnestly desired to provide an apparatus to deposit high quality a-Si film with uniform thickness distribution over large-area substrate at a high throughput.
To deposit thin films such as a-Si film, a parallel-plate (or capacitively coupled type plasma CVD apparatus has been widely used. In this case, the film can be formed on only the surface of one substrate facing the electrode plate. For this reason, to simultaneously deposit films on two substrates in a deposition chamber, the film can be formed at most on both substrates by arranging two electrodes in the chamber forming two discharge regions. There is also an idea of multi-zone deposition system where the number of discharge regions is further increased. However, it is practically very difficult to realize this system because the system has disadvantages due to its complex structure and low maintainability.
In addition, large area solar cells having a desired characteristic can be hardly manufactured since the film thickness uniformity is seriously lowered with the increase in the size of the substrates.
A variety of investigations have been made to observe plasma with uniform density over large area substrates in order to deposit thin films with uniform thickness distribution. However, it is very difficult for the parallel-plate type electrode system to generate uniform plasma over a large area substrate as the electrode becomes large with the substrate. This is attributed to the essential difficulties of the parallel-plate type electrode system, as will be mentioned below.
First, this system requires a precise arrangement of two electrode plates with a prescribed distance all over the electrodes to generate plasma with uniform density, which is practically difficult as the substrate becomes large.
In addition, as the electrodes are enlarged, standing waves tend to appear on the electrode surfaces, which causes non-uniformity of plasma density. This non-uniform plasma distribution becomes more noticeable when higher frequencies such as in the VHF band is employed. For these reasons, the upper limit of the substrate size has been thought to be, for example, 0.3 m×0.3 m when the high frequency of 80 MHz is employed (U. Kroll et al. and Mat. Res. Soc. Symp. Proc. Vol, 557 (1999), p121-126).
Under such a circumstance, other type of plasma CVD method using inductively coupled type electrodes has been proposed. This method is absolutely different in the mechanism for maintaining the discharge from the capacitively coupled type plasma CVD method. This method does not require precise arrangement of electrodes, and high-density plasma can be obtained using the excitation frequency in the VHF band which is advantageous for depositing high quality a-Si film at high deposition rate. The plasma CVD apparatus using inductively coupled type electrodes are exemplified in Japanese Patent Laid-Open 4-236781 that employs a ladder-shaped electrode and in Japanese Patent No. 2785442 that employs a zigzagged-folded electrode.
During the investigations on a variety of inductively coupled electrodes including the above-mentioned electrodes, the present inventors have found that as the inductively coupled electrodes such as the ladder-shaped or zigzag-folded electrodes become larger, the current flowing on the electrodes tends to vary with the positions and standing waves appear at unexpected positions. In short, it was found difficult to create uniform plasma to cope with the large area substrates so far as the electrode structures of the prior art are employed.
Accordingly, the present inventors carried out fundamental investigations on the plasma homogenization using the inductively coupled electrodes and developed several electrode structures that positively utilize the standing waves which caused the uniformity to deteriorate in the prior art inductively coupled electrodes. Here, for instance, a U-shaped electrode was used, which had a power feeding portion at one end and a grounded portion at the other end. The distance from the turning portion to the feeding portion and the grounded portion were set to be a half wavelength of the high-frequency wave to establish the standing wave at predetermined position over the electrode (Japanese Patent Application No. 11-255219). When plasma was generated to form thin film in this configuration, the film thickness distribution obtained was such that the film thickness decreased from the feeding portion toward the turning portion, then increased to show a maximum, decreased again. This distribution is thought to originate in the interaction of the attenuation of high frequency power and the effect of standing waves. Since this film thickness distribution is reproducible, the idea is to obtain thin films with uniform thickness distribution by using only the region with desired uniformity of film thickness of about the same size as the substrate.
Since this film forming method utilizes the portion of the electrode where the uniform plasma density is generated, the electrode becomes longer than the substrate, and thus the apparatus itself becomes larger. On the other hand, a smaller apparatus is strongly requested from the viewpoints of the floor space to be installed, the maintainability, and the cost. Accordingly, the electrode structure and apparatus that can generate the uniform plasma in longer region along the electrode are inevitable to comply with the requests.
In addition, in order to continuously perform stable thin film formation using a plasma CVD apparatus, it is necessary to periodically carry out cleaning etc. to remove films deposited, such as on the inner wall of chamber before the deposited films peel off. However, since the plasma density in the vicinity of the power feeding portion was very high in the case of the U-shaped electrode structure, a large amount of the film deposited on the wall near the power feeding portion. This necessitated more frequent cleaning treatments.
Under such circumstances, the present invention aims at providing a plasma CVD apparatus and method which can form high quality thin films having an excellent film thickness uniformity on larger substrates. That is, the objective of this invention is to provide the electrode structure and the power supply method which make it possible to expand the uniform plasma region in the longitudinal direction of the electrode, and thereby, to realize a plasma CVD apparatus and method which enable to form thin films having an excellent uniformity on a larger substrate using the same size apparatus as that of prior art. Another object of the invention is to provide a plasma CVD apparatus and method that make it possible to form such thin films at a high throughput. Still another object of the invention is to provide a high-productive plasma CVD apparatus and method by suppressing the film deposited, such as on the inner wall of apparatus, to extend cleaning cycle.